1. Field of the Invention
This invention relates to a slide fastener, and more particularly to a fastener stringer having a continuous plastic filament spirally formed and woven into an edge of a stringer tape during the weaving thereof.
2. Prior Art
There have been proposed a number of slide fasteners or zippers of the type described, which comprise a woven stringer tape and a plastic filament woven into a longitudinal edge of the tape, the filament being usually formed into a helical coil structure consisting of a row of successive elongated loops or convolutions each having a coupling head, an upper and a lower leg and a connecting portion.
Most of the known fasteners, however, have failed to meet with the requirements for the success of the so-called woven slide fasteners which largely depends on one hand, upon the positional stability of the filament, i.e. coupling fastener element, with respect to the stringer tape against external stresses, and on the other hand, upon smooth coupling and uncoupling of the opposed stringers by the action of a reciprocating slider.
When the filament is formed into a row of helically coiled coupling loops and mounted on a stringer tape, such loops would usually present a cross-sectionally oval shape which is not complementary with the inner guide surfaces of a slider and hence would often interfere with the sliding movement of the latter during opening and closing of the fastener stringers. With a view to eliminating this difficulty, the coupling loops were covered by covering threads over their upper and lower surfaces including their heel portions in such a manner that the covered portions of the loops present a transverse cross section complimentary in shape with the guide channel of the slider, as typically disclosed in Japanese Patent Publication Nos. 38-20175 and 47-15373. However, such prior art devices have a common drawback in that due to the covering threads being soft and bulky in nature, the slider to be used for this type of stringer is required to be built slightly smaller than the over-all dimensions of the covered coupling loops taking into account the contraction of the covering threads which results from engagement with the slider. This in turn creates an increase in the resistance of the covering threads to sliding movement of the slider, resulting in disagreeable, sometimes interrupted coupling or uncoupling engagement of the cooperating stringers.